A DC power supply unit converts a DC supply voltage, such as that produced from a battery, into a stabilized output voltage by means of a DC voltage converter. In order to reduce voltage fluctuations, an input storage capacitor is typically placed ahead of the DC voltage converter. When the capacitor is completely discharged and the power supply unit is first switched on, a high inrush current flows into the capacitor to charge it. Unless compensated for, this high inrush current causes voltage dips in the DC voltage produced from the battery. Such voltage dips can cause other circuitry energized by the battery to operate incorrectly.
It is also known to place a large capacitor at the output of the DC voltage converter. The large capacitor at the output of the DC voltage converter stores charge that is then utilized under transient conditions. For example, DC converted (or regulated) voltage can be used to power the transmitter circuits of radio frequency (RF) communication equipment. In some RF circuits, such as in cellular telephones, the transmitter RF circuitry operates in pulsed mode, wherein the transmitter must deliver high bursts of power at intermittent times. If the transmitter operates is a pulsed mode, there are times when it must deliver high bursts of power. These large bursts of peak power can cause intermittent excessive battery drain, and therefore voltage dips. One known arrangement employs a large capacitor to store a charge between pulse intervals. The energy stored in the capacitor is used to deliver the peak power required by the transmitter. However, because these large output capacitors have a large capacitance, a large DC charging current (inrush current) is drawn from the capacitor when the input supply is initially switched on. This large inrush current can cause the input battery voltage to dip. The large inrush current can also damage the components of the DC voltage converter. Such voltage dips can cause other circuitry energized by the battery to operate incorrectly. Further, the large inrush current can damage the components of the DC voltage converter connected between the DC voltage supply and the large output capacitor.
Consequently, there is a need to limit the inrush current that can be drawn from the DC voltage supply until input storage/filtering capacitance presented to the DC voltage supply is charged. In addition, there is a need to limit the inrush charging current until the large output capacitance used as a supplemental voltage source during pulsed transmission is charged.